The present invention relates generally to rotary machines, and more particularly, to methods and apparatus to facilitate fluid flow within the rotary machine by reducing fluid leakage losses and fluid mixing losses within the rotary machine.
Rotary machines, such as gas turbines, are used to generate power for electric generators. A gas turbine has a gas path which typically includes, in serial-flow relationship, an air intake (or inlet), a compressor, a combustor, a turbine, and a gas outlet (or exhaust nozzle). Compressor and turbine sections include at least one row of circumferentially spaced rotating buckets or blades positioned within a housing.
Turbine efficiency depends at least in part on a radial clearance or gap between tips of the rotating buckets and a shroud coupled to the surrounding housing. The clearance is needed to avoid contact or rubbing between the bucket tips and the shroud which results in a design limitation for the size of the clearance. If the clearance is too large, enhanced gas flow may leak through the clearance gaps, thus decreasing the turbine's efficiency. Leakage flow, either out of the flow path or into the flow path, from an area of higher pressure to an area of lower pressure, is generally undesirable. If the clearance is too small, the rotor bucket tips may undesirably contact/rub the surrounding shroud during certain turbine operating conditions, which may also decrease the turbine's efficiency. To accommodate for the design limitation of the clearance gap, some known turbines utilize honeycomb and/or labyrinth seals with the shroud and/or bucket to reduce leakage flow through the clearance gap.
FIG. 1 is a cross-sectional view of a known shroud 10 having a seal 12 that can be used with a known gas turbine 14. Known turbines 14 include seals 12, such as honeycomb and/or labyrinth seals, to reduce flow through a gap 16. More particularly, known labyrinth seals 12 include a tortuous path defined by longitudinally spaced-apart rows of labyrinth seal teeth 18 that seal against high-pressure differentials that may be present in the turbine 14. However, the configurations of some known labyrinth seals 12 may induce fluid mixing losses and/or flow leakage losses of the gas on an exit side 20 of rotating buckets 22 that may adversely affect the efficiency of the turbine 14. More particularly, flow paths of leakage flow 24 for some known seals 12 are sometimes not aligned with a subsequent nozzle 26, wherein misalignment increases the re-circulation of leakage flow 24 within the bucket exit side 20 subsequent labyrinth seal 12. The re-circulation of leakage flow 24 may mix with main flow 28, which may also adversely affect turbine efficiency.